Conventionally, polyolefin-based resin foam sheets have been used in a wide range of utility, and examples of this utility may include sealing materials for electronic appliances in addition to a base material of an adhesive tape and a base material of a patch. In these utilities, a polyolefin-based resin foam sheet is required to have a reduced thickness, and flexibility.
As such a polyolefin-based resin foam sheet, Patent Document 1 has proposed a porous sheet with flexibility and mechanical strength improved by adding an inorganic filler to a polyolefin-based resin to form a sheet, and stretching this sheet to render it porous.
However, since it is difficult to realize a high expansion ratio of the porous sheet, the sheet has insufficient flexibility. Further, since the porous sheet has not been subjected to crosslinking treatment, stretching treatment can not be performed at a melting point or more of a polyolefin-based resin constituting a porous sheet; therefore, there is a problem that distortion is occurred in the porous sheet during stretching treatment, and the porous sheet is shrunk during use.
In addition, as a polyolefin-based resin foam sheet, Patent Document 2 has proposed a crosslinked ethylene-based resin foam which comprises a super density polyethylene resin having a predetermined density and a foaming agent, and is obtained by expanding a sheet which has been irradiated with an electron beam of 1 Mrad or less.
However, the crosslinked ethylene-based resin foam has insufficient flexibility and heat resistance. It is considered that the flexibility is improved by increasing an expansion ratio, but there is a problem that when the expansion ratio is increased, the resulting foam is thick.
Patent Document 1: JP-B 7-64942
Patent Document 2: JP-B 6-76505